1. Field of the Invention
The present invention relates to a pipe for supplying fuel to an engine in a gasoline direct-injection engine system or a diesel engine system, and in particular to a pipe having resistance to inferior fuel containing a corrosive component.
2. Description of the Related Art
In recent years, in a gasoline direct-injection engine system whose development has been advanced in an automobile industry for the purpose of reducing environmental load according to fuel consumption improvement a pipe for supplying fuel (gasoline) to a direct-injection rail of an engine from a fuel tank via a pump has been demanded to have pressure resistance and air tightness higher than those of an existing multi point injection engine (see Japanese Patent Application Laid-Open No. 2006-152852). Further, in a diesel engine system, a fuel injection apparatus having a configuration where an injector has been coupled to each cylinder of a high-pressure fuel pipe branched from a common fuel supply passage has been demanded to high pressure resistance and high air tightness to the high-pressure supply pipe (see Japanese Patent Application Laid-Open No. 2002-54354). In addition, a pipe having pressure resistance even to an internal combustion engine using fuel containing much corrosive component such as represented by alcoholic fuel derived from biological materials (hereinafter, referred to as “corrosive fuel”) has been demanded.
Conveying pipes of fuel used in the gasoline direct-injection engine system and the diesel engine system as described above has most adopted products obtained by performing various plastic workings (such as pipe-end forming working, bending working) or joining working (such as brazing working) to a stainless material have been most adopted, as a specification having various properties, such as the above-described pressure resistance, air tightness, and corrosion resistance.
On one hand, for the existing multi point injection engine, a fuel conveying pipe adopting a steel pipe of inexpensive low-grade carbon steel or the like rather than the stainless pipe has been proposed (see Japanese Patent Application Laid-Open No. 2012-26357). Especially, for the purpose of obtaining high resistance to corrosive fuel, the steel fuel conveying pipe is applied with an inner surface treatment and/or an outer surface treatment excellent in resistance to corrosive fuel, including one where a Ni-plated layer has been formed on an inner surface of the steel pipe and an anti-rust film layer composed of a Zn-plated layer or a Zn-based alloy-plated layer has been further provided on the Ni-plated layer, and one where a Zn-plated layer or an Zn-based alloy-based plated layer has been further provided on an outer surface of the steel pipe.
However, in the conventional stainless pipe which has been applied with the plastic working or the joining working, there is such a concern as a stress corrosion cracking (SCC) during plastic working, intercrystalline corrosion or sensitization due to thermal influence during joining working (rising the degree of risk of occurrence of SCC), mechanical property (strength) lowering or the like. In the case of the steel pipe, it is thought that a burrier function is added by applying protection plating of Ni series, Cr series or the like more electrically excellent than steel of a base material, but it is necessary to adopt a film having a substantial film thickness/layer thickness (level of several tens μm) or change a plurality of films to a multilayer/alloy them in order to completely exclude a defect such as a pinhole which may cause corrosion, which results in increase of cost. Alternatively, it is thought to add a sacrifice anticorrosion by applying sacrifice plating of Zn series, Al series or the like electrically inferior to the steel of the base material, but there is such a concern that Zn or Al which is an anti-rust component is eluted in corrosive fuels as ions since the sacrifice plating has a sacrifice anticorrosion, so that the eluted ions raise various adverse effects at various sections of an internal combustion engine.
Further, for the steel fuel conveying pipe which has been applied with an inner surface treatment and/or an outer surface treatment excellent in resistance to corrosive fuels for the purpose of obtaining high resistance to corrosive fuels, for example, when a means of compensating for corrosion resistance inside the pipe with sacrifice anticorrosion of Zn plating adopted by elution of Zn which is an anti-rust component is suppressed up to extremity by forming a Ni-plated layer with a desired layer thickness on an inner surface of the pipe and applying Zn plating on an outer surface of the pipe to a portion of the inner surface of the pipe, there is such a problem that sacrifice anticorrosion will be lost when whole of the Zn plating is eluted. In addition, when non-electroplating of NiP has been used in the Ni-plated layer, because the NiP plating is generally a hard film, there is a possibility that a film crack occurs by working after the plating. Therefore, the non-electroplating of NiP must be performed after working to a product shape, cost increase is thus forced.
Incidentally, as a steel strip (including a steel plate) excellent in corrosion resistance and plating adhesion, a Ni-plated steel strip with high corrosion resistance where the Ni-plated layer has been partially or wholly formed as an Fe—Ni diffusion layer and an Fe exposure ratio in a surface layer has been set to 30% or less has been proposed (Japanese Patent Application Laid-Open No. H06-2104), but the Ni-plated steel strip with high corrosion resistance cannot be said to be sufficient as a material for a fuel conveying pipe which is a subject to be improved by the present invention because in a fuel conveying pipe where a cleanliness factor of an inner surface thereof is specified, rust is under more severe control.
The present invention has been made in view of the conventional problems, and it is to provide a steel fuel conveying pipe with a high quality having high resistance to corrosive fuel, having followability and allowing working after plating in a pipe for supplying fuel to an engine in a gasoline direct-injection engine system or a diesel engine system.